1. Field of Invention
The present invention relates to a lithographic apparatus and method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”- direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
When a radiation beam is incident upon the patterning device, the patterning device is heated by the radiation beam. For example, heating may occur due to absorption of some of the energy of the radiation beam into the patterning device. Heating of the patterning device can lead to expansion of the patterning device. If the expansion of the patterning device is not taken into account, a pattern may be incorrectly applied to the substrate. For instance, overlaid patterns applied to the substrate may not totally overlap with one another, such that there is an overlay error between successively applied patterns. This may occur due to the patterning device having a different temperature (and therefore having a different degree of expansion) for each successive exposure. In known arrangements, the expansion of the patterning device may be taken into account by providing the patterning device with alignment marks. For instance, the alignment marks may be located in or around an area of the patterning device which defines an image field. By detecting changes in the positions of the alignment marks, the degree of expansion of the patterning device, and in particular the image field, can be determined. Elements of the lithographic apparatus, for example lenses of a projection system of the lithographic apparatus, can be adjusted to compensate for the expansion of the patterning device, thereby ensuring that patterns are correctly applied to the substrate.
In known arrangements, lenses of the projection system are adjusted (e.g. moved) before each exposure. The lenses may be adjusted to take into account a uniform expansion of the patterning device, the adjustment being arranged to provide a correction (i.e., a change) of a magnification factor (or de-magnification factor) of the projection system required to reduce the size of pattern features applied to the substrate to a desired size. During application of the pattern to the substrate (using a static or scanned exposure), the lens elements are not adjusted.
In some known arrangements, the expansion of the patterning device is assumed to be uniform due to it being heated by the radiation beam. In arrangements where the expansion is assumed to be non-uniform, the magnification correction obtained by adjustment of the lenses of, for example, the projection system is uniform (in a plane of the image field) and remains unchanged during an exposure. This means that the adjustment of the lenses of the projection system may not accurately reflect an actual and non-uniform expansion of the patterning device within, for example, an exposure field. This may lead to for example, overlay errors in patterns successively applied to the substrate.